In vitro systems are being used to investigate the mechanisms by which cyclic nucleotides alter cell function and to explore the mechanism of action of agents whose effect on cell function is mediated by cellular accumulation of cyclic nucleotides. BIBLIOGRAPHIC REFERENCES: Gardner, J. D., Aurbach, G. D., Spiegel, A. M. and Brown, M. E.: Receptor function and ion transport in turkey erythrocytes. In Greep, R. O. (Ed.): Recent Progress in Hormone Research, Vol. 32, Proceedings of the 1975 Laurentian Hormone Conference, New York, Academic Press, pp 567-595, 1976. Brown, E. M., Gardner, J. D. and Aurbach, G. D.: Direct determination of ligand interactions with beta-adrenergic receptors on intact turkey erythrocytes: correlation of binding with biological activity. Endocrinology, 99: 1369-1376, 1976.